It has been reported previously that administration of the same dose of mifepristone can produce widely varying blood serum levels in different patients. The varied blood serum levels can result in some patients not receiving an efficacious dose of mifepristone. For patients suffering from a mental disorder, the blood serum levels need to be maintained at about 1300 ng/mL. For patients suffering from Cushing's syndrome, it was surprisingly discovered that blood serum levels need to be maintained at a level of at least about 1631 ng/mL for a therapeutic response. Thus, a method for ensuring that the blood serum levels of mifepristone remain in an efficacious and safe range is needed for patients suffering from Cushing's syndrome.